In rock bolting, bolts or wires are fastened by means of concrete within holes drilled in the rock for the reinforcement of the rock in mines and excavations.
Swedish Pat. Specification No. 7901616-8 discloses a rock bolting device in which a concrete feeding pipe is attached to a carriage which is displaceable along a supporting beam and by means of which the feeding pipe can be pushed into and withdrawn from a hole drilled in the rock.
Concrete is fed into the drill hole by means of this known device by pushing the feeding pipe up to the bottom of the hole and by feeding concrete into the hole simultaneously as the feeding pipe is drawn outwards within the hole. For the withdrawal of the feeding pipe, the operation of a displacing motor moving the carriage of the feeding pipe is positively synchronized with the operation of the motor of the concrete feeding pump so that at the same time as the feeding pipe moves outwards within the hole, concrete is fed into the hole through the feeding pipe in an amount which is estimated to be sufficient for filling the hole.
However, this way of feeding concrete is disadvantageous in that it does not reliably ensure that the drill hole really will be filled with the used amount of concrete. This is because there may be crushes and cracks in the rock, and the concrete may penetrate into these cracks and crushes when it is fed into the hole. Since the withdrawal of the feeding pipe is synchronized with the concrete feed, the result in such a case is that the hole is filled incompletely as the concrete mass intended for the hole penetrates into the crushes and cracks. On the other hand, it cannot be ascertained whether the crack has been filled, even incompletely, with the concrete mass. On account thereof, the placement of concrete about the rock bolt may be incomplete.
Swiss Pat. Specification No. 615,245 discloses a method for feeding concrete into a drill hole. This method is based on the idea that the concrete fed into the hole presses the feeding pipe outwards in the hole in proportion as the hole gets filled with concrete. However, the realization of this method requires that a flexible porous hose is pushed into the drill hole together with the feeding pipe so that the hose surrounds tightly the point portion of the feeding pipe, and the concrete flows within the hose. By means of this method, it is not possible to fill cracks and crushes, not even partially.
It has been suggested in Finnish Pat. Application No. 832,124 that the filling pressure of concrete in the drill hole should be utilized for positively drawing the feeding pipe out of the hole by means of a driving device, depending on the filling pressure of concrete prevailing in the hole at each particular moment. It is thereby ensured that not only the drill hole but also the cracks and crushes coming in the way of the hole can be filled with concrete. When the bolts are long, e.g. over 4 meters, the weight of the feeding pipe after it has been filled with concrete, the friction between the pipe and the hole, and the difference in the diameters of the pipe and the hole prevent the feeding pipe from acting as a piston for pushing the concrete. It is thereby necessary to adjust the withdrawing velocity of the feeding pipes so that it corresponds to the production rate of the pump, which implies uncertain factors.